Rotary encoders fall into three general categories, optical, magnetic and mechanical contact. Mechanical contact encoders require brush contacts, which are susceptible to wear and need replaced regularly. Magnetic encoders are contactless, but cannot be operated in some environments where the magnetic fields might interfere with other equipment. In general, optical encoders are the most widely used as they provide the highest accuracy and the best reliability per unit cost.
Within the field of optical rotary encoders there are two main types. The first is usually known as an incremental encoder and is illustrated in FIGS. 1a and 1b. As shown in the plan view of FIG. 1a, the rotary encoder comprises a disk 10 comprising alternating clear and opaque sections 12,14 around its circumferential edge. It is to be appreciated that the sections could be formed at any radial point on the disk, for example around a circle that is closer to the center than is illustrated here.
The disk can be formed from glass, plastic or metal among other materials, and the clear and opaque sections 12,14 can be formed by etching, printing, embossing or any other suitable method. The disk 10 is fixed to a shaft 16 which rotates in use. FIG. 1b shows a side view cross-section of the encoder. The disk 10 is interposed between one or more light sources 18 and one or more corresponding sensors 20 with associated circuitry, and positioned such that the light sources 18 and sensors 20 are arranged on either side of the circumferential edge comprising the clear and opaque portions 12,14.
As the disk 10 rotates, the intensity of light incident on the sensors varies as the clear and opaque patterns 12,14 pass the under the light sources 18 in sequence. The measured intensity is amplified or fed into a comparator to produce a sign wave or digital square wave. The pulses of this output are then counted by the circuitry associated with the sensors 20 to give positional information as the intensity of the light varies.
In order to yield an absolute angular position, a known a reference point must be provided. This can be in the form of a single opaque section 22 on an outer track of the disk 10 or by use of a separate mask component as is well known in the art. In one embodiment, a first light source and sensor pair is provided for detecting the varying intensity of the track comprising the alternating clear and opaque parts 12,14, and a second light source 18a and sensor 20a pair is provided for the detection of the reference point 22.
This type of optical rotary encoder requires only a small number of sensors and light sources. However, if the accuracy of the encoder is to be increased, this means that the number of etched lines need to form the alternating clear and opaque portions 12,14 must be increased. The increased complexity of etching raises the cost of the disk 10. A further disadvantage of this type of sensor is that if power is lost from the device, the stored absolute positional reference information is lost.
The second type of optical rotary encoder is an absolute encoder, an example of which is illustrated in FIG. 2. A similar disk 30 is fixed to a rotatable shaft 32. However, in this case the patterns formed on the disk 30 are more complicated, comprising a number of tracks T1-T5 which are formed as non-overlapping annuli each of which is concentric with the central axis of rotation of the disk 30 which runs perpendicular to its surface. Each track T1-T5 comprises alternating clear and opaque portions 34,36. These portions are arranged such that successive angles of rotation yield unique patterns of alternating clear and opaque portions across the five tracks T1-T5. The code formed from the pattern can for example be a binary code, or a gray code implementation such as that illustrated, where only one bit of code varies between successive segments. This helps minimize errors caused when the angle is close to the borderline between segments of the circle, or errors due caused by the alignment errors of the light sources and sensors. The code generated can give the position of the shaft, and successive readings can be used to calculate its speed and direction.
Light sources 38 and sensors 40 are provided as before, but in this case a separate light source 38 and sensor pair 40 is provided for use with each track. Each track provides one bit of code. In the illustrated example, a five bit code is generated by the five tracks T1-T5. This means that the maximum number of angles that can be detected is 25-1, that is, thirty-one. The disk 30 is thus divided into thirty-two different segments. When the disk is at a particular angular location a unique code is generated which gives direct absolute positional information from start up.
To increase the accuracy of such an encoder, additional tracks are needed which increases the complexity of etching required to fabricate the encoder disk. It is to be appreciated that the illustrated example is not limiting, and that a rotary encoder of this type will typically comprise more tracks, or could comprise less.
As mentioned, the advantage of this type of encoder is that it gives an absolute positional information from start up. However, as compared with the incremental encoder, it requires an increased number of sensors and light sources. The production of the etched disk is also relatively expensive as compared with the incremental encoder.
It is therefore an object to provide an optical rotary encoder which is more simple to manufacture. Other objects will be apparent from the context of this application as a whole.